A thorough understanding of the abilities of normal- hearing and hearing-impaired individuals to process the spectral-temporal properties of speech is needed. In particular, the relation between the basic capabilities of the auditory system to resolve the acoustic details of speech and the perception of speech in normal discourse is poorly understood. Our research program directly addresses this need in three lines of research within a general theory of vowel perception. Listeners are selected to be typical of the population with either normal hearing or mild-to-moderate high-frequency sensorineural hearing impairment. The first line of research will determine the weighted contribution of three salient acoustic properties to the classification of American English vowels. This research explores the use of these properties in the identification of natural vowels in optimal conditions and in several conditions of distortion, including vowels presented in noise and vowels spoken with a foreign accent. The second line of research uses psychophysical techniques to determine listeners' thresholds for discriminating acoustic differences between vowels, including modeling of the peripheral auditory processing of these complex sounds. In addition, this research investigates the manner in which discrimination performance is degraded as experimental tasks change from those used in psychophysical procedures to those that simulate communication with natural sentences. In the third line of research, the relation between discrimination abilities and vowel classification in fluent speech will be determined. These experiments will describe systematically the relation between peripheral capabilities to process vowels and the more central processes of vowel classification in ordinary discourse. Results will also make major advances towards a new theory of vowel perception. The data from these experiments will contribute to the understanding of the impact of cochlear dysfunction on normal conversation processes.